The present invention relates to a hydraulic drive system equipped in a construction machines such as a hydraulic excavator, and more particularly to a hydraulic drive system including a load sensing control system for controlling a delivery pressure of a hydraulic pump so that a differential pressure between the delivery pressure of the hydraulic pump and a maximum load pressure among a plurality of actuators is maintained at a setting value.
Prior-art hydraulic drive systems each having a load sensing control system (hereinafter referred to also as an xe2x80x9cLS systemxe2x80x9d) are disclosed in, e.g., Japanese Patent No. 2986818 and JP,A 10-205501.
The hydraulic drive system disclosed in Japanese Patent No. 2986818 comprises a fixed displacement hydraulic pump, actuators driven by a hydraulic fluid delivered from the hydraulic pump, a plurality of flow control valves for controlling flow rates of the hydraulic fluid supplied from the hydraulic pump to the respective actuators, and an unloading valve for controlling a delivery pressure of the hydraulic pump so that a differential pressure between the delivery pressure of the hydraulic pump and a maximum load pressure among the actuators (hereinafter referred to also as an xe2x80x9cLS differential pressurexe2x80x9d) is maintained at a setting value.
The hydraulic drive system disclosed in JP,A 10-205501 comprises a variable displacement hydraulic pump, a plurality of actuators driven by a hydraulic fluid delivered from the hydraulic pump, a plurality of flow control valves for controlling flow rates of the hydraulic fluid supplied from the hydraulic pump to the plurality of actuators, a plurality of pressure compensating valves for controlling respective differential pressures across the plurality of flow control valves, and a pump delivery control means for controlling a delivery capacity of the hydraulic pump so that an LS differential pressure is maintained at a setting value. The plurality of pressure compensating valves have respective target differential pressures each set equal to the LS differential pressure.
The hydraulic drive system disclosed in JP,A 10-205501 further comprises a fixed displacement pilot pump driven by an engine along with the variable displacement hydraulic pump, a throttle disposed in a pilot delivery line, and a setting changing means for changing the setting value of the LS differential pressure in accordance with a differential pressure across the throttle. When the engine revolution speed lowers, the setting value of the LS differential pressure is reduced corresponding to the lowering of the engine revolution speed, thereby reducing the flow rate supplied to the actuator. As a result, operability capable of allowing a sufficient quantity of works can be ensured when the engine revolution speed is at the rated revolution speed, and the actuator speed can be adjusted depending on the engine revolution speed, thus resulting in improved fine operability.
With the hydraulic drive system disclosed in Japanese Patent No. 2986818, since the unloading valve controls the delivery pressure of the hydraulic pump so that the LS differential pressure is maintained at the setting value, the LS system can be constructed by using even the fixed displacement hydraulic pump. However, the hydraulic drive system having such a construction cannot adjust the actuator speed depending on the engine revolution speed unlike the hydraulic drive system disclosed in JP,A 10-205501. Hence, if the setting value of the LS differential pressure is set with an emphasis focused on the operability resulting when the engine revolution speed is at the rated revolution speed, fine operability cannot be ensured at a satisfactory level when the engine revolution speed is reduced.
With the hydraulic drive system disclosed in JP,A 10-205501, when the input amount of a control lever of a control lever unit is changed and the flow rate demanded by the flow control valve is also changed, the LS differential pressure is maintained at the setting value by controlling the delivery capacity of the variable displacement hydraulic pump, and therefore response of the hydraulic pump defines response of the hydraulic drive system (i.e., response of a hydraulic excavator when the hydraulic drive system is equipped in the hydraulic excavator). However, since there is a limitation in response of the hydraulic pump, a delay occurs in control of the flow rate supplied to the actuator, causing an operator to feel a time lag in machine movement.
It is an object of the present invention to provide a hydraulic drive system including an LS system, which can ensure fine operability based on setting of the engine revolution speed, can perform flow rate control at a good response, and can realize superior operability.
(1) To achieve the above object, the present invention provides a hydraulic drive system comprising an engine, a first fixed displacement hydraulic pump driven by the engine, a plurality of actuators driven by a hydraulic fluid delivered from the first hydraulic pump, a plurality of flow control valves for controlling flow rates of the hydraulic fluid supplied to the plurality of actuators from the first hydraulic pump, a plurality of pressure compensating valves for controlling respective differential pressures across the plurality of flow control valves, the plurality of pressure compensating valves having respective target differential pressures set in accordance with a differential pressure between a delivery pressure of the first hydraulic pump and a maximum load pressure among the plurality of actuators, wherein the hydraulic drive system further comprises an unloading valve for controlling the delivery pressure of the first hydraulic pump so that the differential pressure between the delivery pressure of the first hydraulic pump and the maximum load pressure among the plurality of actuators is maintained at a setting pressure, and variably setting means for setting the setting pressure of the unloading valve as a variable value that varies depending on a revolution speed of the engine.
Thus, the unloading valve and the variably setting means are provided, the delivery pressure of the first hydraulic pump is controlled so that the differential pressure between the delivery pressure of the first fixed displacement hydraulic pump and the maximum load pressure among the plurality of actuators is maintained at the setting pressure, and the setting pressure of the unloading valve is set as a variable value that varies depending on the engine revolution speed. In the LS system, therefore, an actuator speed can be adjusted depending on setting of the engine revolution speed, and fine operability based on setting of the engine revolution speed can be ensured.
Further, in general, a valve unit operates at a faster response than a hydraulic pump. Therefore, when the flow rate demanded by the flow control valve is changed, the flow rate supplied to the actuator can be controlled at a good response with the delivery pressure of the first hydraulic pump controlled by the unloading valve.
(2) In above (1), preferably, the variably setting means comprises a second fixed displacement hydraulic pump driven by the engine along with the first hydraulic pump, a flow rate detecting valve disposed in a delivery line of the second hydraulic pump, and setting changing means for changing the setting pressure depending on a differential pressures across the flow rate detecting valve.
With that feature, the variably setting means sets the setting pressure of the unloading valve as a variable value that varies depending on the engine revolution speed.
(3) In above (1), the variably setting means may comprise a flow rate detecting valve disposed in a delivery line of the first hydraulic pump, and setting changing means for changing the setting pressure depending on a differential pressures across the flow rate detecting valve.
With that feature, the variably setting means sets the setting pressure of the unloading valve as a variable value that varies depending on the engine revolution speed, without using a special hydraulic pump.
(4) In above (2) or (3), the flow rate detecting valve may be a fixed throttle.
With that feature, the flow rate detecting valve can detect the delivery rate of the first or second fixed displacement hydraulic pump and detect the engine revolution speed with a simple structure.
(5) In above (2) or (3), the flow rate detecting valve may be a valve having a variable throttle built therein and regulating an operating state of the variable throttle in accordance with a differential pressure across the flow rate detecting valve itself.
With that feature, the relationship between the engine revolution speed and the setting pressure of the unloading valve can be freely set. As a result, the setting capable of allowing the actuator supplied flow rate to be adjusted over the entire range of a lever stroke of a control lever unit for the corresponding flow control valve at the rated engine revolution speed can be also maintained in the status in which the engine revolution speed is reduced, whereby saturation during the combined operation can be avoided and more satisfactory fine operability can be obtained.